Priority is claimed to Japanese Patent Application Nos. 10-174183, filed on Jun. 22, 1998, and 11-151198, filed on May 31, 1999, herein incorporated by reference.
1. Field of the Invention
The present invention relates to a method of measuring insulation resistance of capacitors to be used in judgment of conformance and nonconformance of capacitors, etc.
2. Description of the Related Art
In order to judge conformance and nonconformance of capacitors to a standard, a method of measuring insulation resistance of capacitors is known. This known method measures current (charging current) flowing through a capacitor after a direct-current voltage for measurement is applied to the capacitor and the capacitor has been fully charged. As a matter of course, conforming capacitors have less charging current.
Up to now, among such methods of measuring insulation resistance, a measurement method defined in JIS-C 5102 is known. Because this method requires measurement of the current in the state that the capacitor has been fully charged, a measuring time of, e.g., about 60 seconds was required. However, an improvement of productive capacity and quality of electronic parts such as capacitors, etc. is required in accordance with the needs of cost reduction and reliability improvement of electronic equipment. As such, the conventional measurement method, in taking such a long measuring time per capacitor, is not able to fulfill such requirements.
In order to measure insulation resistance of chip capacitors efficiently, a turntable is used. In measurement methods of insulation resistance using a turntable, there is a continuous production that, after capacitors have been advanced past a plurality of preliminary charging stations, the insulation resistance of the charged capacitors is measured one at a time. There is also a batch production that, after a fixed number of capacitors have been fed on a turntable, the turntable is stopped and at the same time preliminary electrical charge is given to a plurality of capacitors, and their insulation resistance is measured thereafter.
FIGS. 1 and 2 are the drawings showing the fundamentals of the above-mentioned two measurement methods. Capacitors 1 are held on a conveying means 2 such as a turntable, etc. at intervals of a fixed space and transported in the direction of the arrow intermittently. FIG. 1 shows a continuous way of production. After a capacitor 1 has been fed at the feeding station SIN, preliminary charge takes place at a plurality of preliminary charging stations of Sp1 To Sp4 every time the conveying means 2 stops, and then a measuring voltage E[v] is applied at the measurement station SM and the insulation resistance is measured by a measuring apparatus 3. After that, nonconforming capacitors are unloaded at the unloading station SNG for nonconforming articles and conforming capacitors are unloaded at the unloading station SG for conforming articles.
FIG. 2 shows a batch production. In the state that a plurality of capacitors 1 are held on a conveying means 2, the conveying means 2 is stopped for a fixed period of time, and a voltage which is the same voltage as the measuring voltage Em is applied at a plurality of measurement stations of SM1 To SM5 for preliminary charge and at the same time the insulation resistance is measured by measuring apparatuses 3. Further, reference numeral 4 represents a power supply of a rated voltage E[v], reference numeral 5 terminals for preliminary charge, and reference numeral 6 measurement terminals.
In the former case, because the electrodes of a capacitor are necessary to be made in contact with the terminals for preliminary charge many times, there is a disadvantage that the electrodes are likely to be damaged. Further, in the latter case, because preliminary charge is given to a number of capacitors 1 and they are measured at the same time, a large power supply unit is required. Furthermore, because multichannel measurements take place, many measuring apparatuses 3 are needed. Also, because one measuring apparatus 3 is switched for measurement, a complicated switching circuit is required. As a result, there was a drawback that the cost of equipment becomes high and the maintenance faces a difficulty.
Accordingly, it is an object of the present invention to present a method of measuring insulation resistance of capacitors in which the number of contact of terminals with a capacitor is able to be reduced and a circuit necessary for measurement is able to be simplified.
In order to attain the above-mentioned object, in a first aspect of the present invention, a method of measuring insulation resistance that the insulation resistance is measured through a charging current flowing through a capacitor when preliminary charge takes place by applying a charge voltage Ep to the capacitor and then a measuring voltage Em is applied is characterized in that self-charge takes place in an open state during the time from the finish of the above-mentioned preliminary charge to the application of the measuring voltage Em.
Here, the principle of a method of measuring insulation resistance according to the present invention is explained. An equivalent circuit of a capacitor such as a ceramic capacitor is composed of capacitance C0, internal resistance r, insulation resistance R0, and the component of dielectric polarization (component of electrostatic absorption) D as shown in FIG. 3. When a direct-current voltage is applied to such a capacitor, its charging characteristic is as shown in FIG. 4. That is, the initial nonlinear charging characteristic A represents a region of charging capacitance C0. The linear charging characteristic C represents a region of charging the component of dielectric polarization D. The characteristic B represents a transition region between them. In FIG. 4, the vertical axis (current) and horizontal axis (time) are of a log scale.
Now, in FIG. 4, the charge is stopped in the course of the linear charging characteristic C and the capacitor is left in an open state, that is, without voltage applied. Then, after a fixed period of time, the charge is restarted. At this time, it has been found that, as shown by a broken line in FIG. 4, although the charging current value goes high once, the value is immediately stabilized on the linear charging characteristic C. After discussion of the phenomenon, the following reasoning was thought about by the present inventors. When charging is started initially, capacitance C0 of a capacitor is charged by a charging voltage. But because it takes time to charge the component of dielectric polarization D, the component D is little charged at the initial step. While the charge is stopped, there is no flow of electric current to or from the outside because the capacitor is in an open state, that is, without voltage applied. During this time, charge (self-charge) of the component of dielectric polarization D by the electric charge in capacitance C0 takes place, and this charge is in progress as if the first electric charge was not interrupted. Further, as capacitance C0 is larger in capacitance than the component of dielectric polarization D, the charging voltage is lowered a little. When the charge is started again, because the charge of the component of dielectric polarization D has already progressed, it is thought that a little charge makes the capacitor stabilized in the desired charging characteristic C.
So, according to the present invention, by self-charge in an open state, that is, without voltage applied during the time from the finish of preliminary charge to the application of a measuring voltage, a capacitor is able to be charged without the electrodes of the capacitor making contact with the terminals many times for the purpose of preliminary charging. Furthermore, when a measuring voltage is applied to the capacitor, the self-charge of which has finished, it is possible to measure normal insulation resistance in a short period of time. Therefore, multiple measuring apparatuses and complicated switching circuit are not required.
As in a third aspect of the present invention, a period T1 for preliminary charge is desirable to be set to be more than time Tb of a charging time A of capacitance C0 plus a transition time B. In this way, capacitance C0 is fully charged. Time Tb is not fixed, and if the preliminary charge voltage Ep is set higher than the measuring voltage Em, the time Tb is able to be reduced. Further, preliminary charge is not limited to one time, and a plurality of preliminary charges is allowed. When preliminary charge of two times takes place, the first and second preliminary charge may be well set to be more than Tb/2 respectively.
As in a fourth aspect of the present invention, an open period T2 for self-charge is desirable to set to be more than time Tc which is needed to reach a charging current value A1 when a rated voltage is applied to the insulation resistance R of a capacitor. In other words, this is to secure the time for the component of dielectric polarization D to be fully self-charged.
As in a fifth aspect of the present invention, a first preliminary charge may be performed at the same voltage as the measuring voltage Em, a secondary preliminary charge may be performed at the same voltage as the measuring voltage Em after a predetermined open period, and the measuring may be performed afterwards. During the open period, a voltage e applied to both ends of the capacitor decreases a little because of the consumption by the insulation resistance Ro or the charge to the dielectric polarization component D. The longer the open period is, the more the voltage decreases. When the measuring voltage Em is applied later, re-charge to the capacitance Co because of the decreased voltage, thereby making it longer to reach to the predetermined charging time A. Thus, in the secondary preliminary charge, the voltage Ep which is the same as the measuring voltage Em is applied thereby reducing the voltage decrease, which shortens the measuring time. Preferably, the secondary preliminary charge is applied just before the charge of the measuring voltage Em.
Further, the secondary preliminary charge also has an effect as to be explained in the following. That is, during the open period the component of dielectric polarization of a capacitor is charged by a voltage e [v] between the terminals of the capacitor. However, because the voltage e [v] gradually diminishes in accordance with the lapse of the open period, e [v] becomes smaller than Em [v]. When a measuring voltage Em [v] is applied, the time required to charge the component of dielectric polarization for the second time is far longer than to charge capacitance C0. For example, in the case of a ceramic capacitor of 1 xcexcF, capacitance C0 takes a few milliseconds as charging time, but the charging time of the component of dielectric polarization D is a few hundreds milliseconds. In order to reduce this voltage drop Emxe2x88x92e [v], the secondary preliminary charge takes place. The secondary preliminary charge in this case may be subjected to after a time of {square root over ((Tc))} has lapsed from the first preliminary charge. The reason is that a voltage drop after the first preliminary charge is made equal to a voltage drop after the secondary preliminary charge and the charge of the component of dielectric polarization D to a voltage different from the measuring voltage Em [v] is prevented.
As in a sixth aspect of the present invention, it is preferably that the charge voltage Ep of the preliminary charge is set higher than the measuring voltage Em, and the charge voltage Ep is set so that the voltage e of both ends of the capacitor decreases equal to or less than the measuring voltage Em in the open period. Namely, as described above, during the open period the voltage e applied to the both ends of the capacitor decreases a little because of the consumption by the insulation resistance Ro or the charge to the dielectric polarization component D. Since the standard voltage which is similar to the measuring voltage Em is applied in the preliminary charge, when the open period gets longer, it takes time for the charged current to gather because of the voltage decrease of the voltage e at measuring thereby causing the delay in measuring the leaked current. Thus, by setting the charge voltage Ep higher than the measuring voltage Em, the voltage decrease of the voltage e is suppressed and the leaked current can be measured in a short period by gathering the charge current quickly when measuring.
However, when the preliminary charge voltage Ep is set too high, the reverse current may flow when measuring since the voltage e applied to the both ends of the capacitor is higher than the measuring voltage Em. Namely, insulation resistance is measured higher than in actual, thereby being misjudged that it is a good product, although it is a bad product. So, in the present invention, the reverse current is prevented from flowing by setting the charge voltage Ep so that the voltage e applied to the both ends of the capacitor decreases equal to or less than the measuring voltage Em in the open period.
As in a seventh embodiment of the present invention, the charge voltage Ep of the preliminary charge may be set higher than the measuring voltage Em, and the secondary preliminary charge may be performed at the same voltage as the measuring voltage just before the measuring in the predetermined open period. Namely, when the high voltage preliminary charge is performed as in the sixth embodiment, the reverse current may flow when applying charge because there is such a case that the voltage decrease in an open period is small and the voltage applied to both ends is higher than the measuring voltage when measuring. Thus, the flow reverse current when measuring can be prevented by performing the secondary preliminary charge at the same voltage as the measuring voltage Em just before measuring and by correcting the voltage e of both ends of the capacitor to the measuring voltage Em.
As in an eighth aspect of the present invention, the first preliminary charge may be performed at the voltage Ep1 which is higher than the measuring voltage Em, the secondary preliminary charge may be performed at the voltage Ep2 which is lower than the voltage Ep1 and higher than the measuring voltage Em after a predetermined open period, and the measuring may be performed after a further predetermined open period.
In this case, since two times of preliminary charge are performed at the voltage Ep1 and Ep2 which are higher than the measuring voltage Em, although the open period is short, the charge to the dielectric polarization component D can be proceeded fully.
Preferably, since the reverse current is prevented, the secondary preliminary charge voltage may be set so that the voltage e applied to both ends of the capacitor decreases equal to or less than the measuring voltage Em during the open period.
When a high voltage preliminary charge is performed as in the seventh and eighth embodiments, as in a ninth embodiment, an electric discharge can be performed just after the high voltage preliminary charge. Namely, when the high voltage preliminary charge is performed longer than necessary, the charge voltage becomes too high and the reverse current flows. In such a case, when measuring is performed, a characteristic of a good product is shown although it is a bad product. Therefore, it can be prevented that the charge voltage becomes too high by performing electric discharge after the high voltage preliminary charge.